Blue whales are impressive creatures. Stretching up to 90 feet long and weighing over 300,000 pounds, these gentle giants often migrate hundreds of miles between their summer feeding grounds and winter breeding grounds. It’s no surprise, then, that the whales are pros at long distance communication, producing calls that can be heard up to 1,000 miles away.

Scientists at the Monterey Bay Aquarium Research Institution (MBARI) and Stanford University are studying those calls to gain insight into how blue whales live and hopefully, how we can better protect them. It turns out these creatures have an entire language—four different types of calls creatively named A, B, C, and D calls—that researchers can listen to by sticking microphones on the seafloor or on the backs of the animals.

The calls are, as Stanford PhD student and blue whale acoustics aficionado William Oestreich put it to Earther, “stereotypical.”

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“Every time you detect one, it basically sounds exactly the same,” he said.

That doesn’t make them any less impressive. “It is very rare to have such a long-running, continuous acoustic record of blue whale calls,” Oestreich told Earther, referring to MBARI’s long-term ocean listening project which has been logging whale calls since 2015 with a hydrophone that sits 18 miles offshore. And despite recording from about 3,000 feet underwater, some of the calls are astoundingly crisp and clear.

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For instance, the recordings below feature A and B calls captured by the hydrophone. The A calls consist of a series of low frequency pulses pitched up 100 percent; the B call, which has been pitched up 400 percent to make it audible, is a two-part tonal that’ll make you feel like you’re standing next to a subwoofer at a dubstep show. (For the full effect, it’s worth popping on a good pair of headphones to listen to these.)

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The C call is similarly stereotypical—a single tone at a very low frequency that’s hard to hear—but the D call is “kinda its own beast,” Oestreich said. He described it as a down-sweeping “groaning sound” that starts with a higher pitch and ends on a deep bass note. And unlike the other calls which sit within a narrow frequency range, the D call can be more variable.

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Here’s a recording that captures a few examples, pitched up 250 percent:

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In its almost four years at sea, the MBARI’s listening station has amassed tens of thousands of blue whale calls, Oestreich said. But while that’s a lot of data, it’s missing some key context. How many whales are singing these haunting tunes, and what sorts of behaviors are they engaged in while they’re doing so?

A complementary effort led by one of Oestreich’s advisors—Stanford marine biologist Jeremy Goldbogen—seeks to add that context. Goldbogen and his team are attaching scientific instruments to blue whales in Monterey Bay not only to record calls, but capture information on their movements, feeding and diving behavior, as well as video.

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“That can allow us to understand when, and where, [and] in what circumstances, are they using these different call types,” biological oceanographer John Ryan, who leads the MBARI’s underwater listening effort, told Earther.

This tagging effort, which has been ongoing in Monetary Bay since 2017, doesn’t capture the same volume of calls that the passive listening can. But combining the two is yielding some tantalizing insights.

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“The most striking thing that jumps out is there’s a lot of animals that, at least when the tag is on them, were not talking at all,” Oestreich said. “Many of them aren’t calling.”

Another curious observation—although this is still tentative—is that the blue whales off California’s coast will often pair up to feed together, and those that do so have “produced a lot of calls,” Oestreich said. “So that’s an interesting kind of high level first cut thing. But in terms of saying it was a mating or feeding call, this is the stuff we’re still working out.”

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Beyond the basic behavioral insights, the research could prove useful for conservation. Although blue whales have been on the rebound since being decimated by commercial whaling in the early 20th century, they’re still considered endangered across the world. Today, the biggest threats are fishing gear entanglement and ship strikes, according to NOAA. Various organizations, including the Benioff Ocean Initiative, are exploring the possibility of setting up an acoustic monitoring and notification system that would alert ships when whales are in the area, but there are still a lot of unknowns that could impact its effectiveness—like how frequently the whales call, and whether certain groups are chattier than others.

These questions are enough to keep scientists busy tagging whales and tuning up their underwater mics for years. “If only we could share their thoughts,” Ryan said. “The best we can do is make more in depth observations of their behavior.”